1. Field
The following description relates to optical link monitoring technology that detects faults of an optical link used as a transmission medium and monitors performance of the optical link.
2. Description of the Related Art
Optical link monitoring technology, which detects faults of an optical link and monitors performance of the optical link, is technology for securing reliability of an optical network. In a long-distance signal transmission, the optical link monitoring technology needs to monitor a distance as long as possible. Since functions of monitoring faults and performance are necessarily required in a subscriber network, in a subscriber network with a point-to-multipoint architecture, research on optical link fault detection in a point-to-multipoint architecture that connects one optical line terminal (hereinafter referred to as ‘OLT’) and a plurality of optical network units (hereinafter referred to as ‘ONU’) has been underway.
Research on such methods includes a method of identifying ONUs having a fault using a different monitoring wavelength per ONU with a tunable laser, and a method of identifying ONUs having a fault that slices amplified spontaneous emission (ASE) using an erbium-doped fiber amplifier (EDFA) and a fiber Bragg grating (FBG) and allocates the sliced ASE to each ONU. Recently, a method of monitoring each ONU that assigns a special optical fiber manufactured to have different Brillouin frequency characteristics to each ONU is proposed.
In order to increase reliability of systems or networks and effectively manage optical links, functions of accurately diagnosing and analyzing causes of faults are very important. An optical time domain reflectometer (hereinafter referred to as ‘OTDR’), which is a type of optical link fault monitoring devices that measures loss or attenuation characteristics of optical fibers, using backscattered signals and backreflected signals returned when a short pulse is applied to an optical fiber.
Methods of improving performance of the OTDR have been studied in various aspects. As part of such research, there are technologies for improving, for example, resolution or distance accuracy, of a pulse-based OTDR. For example, a coherent OTDR and a code-based OTDR can improve resolution of the OTDR.
The pulse-based OTDR, which measures optical intensity of signals reflected from the is optical fiber by applying optical signals to the optical fiber and schematizes traces with respect to a distance, may identify a fiber cut of the optical fiber, but it is difficult to precisely identify causes of faults due to non-reflective events, for example, a bending or a connector loss. In order to overcome these limitations, methods using a Brillouin OTDR and a dichroic reflective filter are proposed so that it is possible to identify a bending or a connector loss that could not be identified in the OTDR. However, performance of the Brillouin OTDR is unstable since it is easily affected by an environment, for example, temperature or pressure. Moreover, since the method using the dichroic reflective filter requires a specially designed filter, it is expensive and difficult to implement.